vii LAW AND PRINCIPLE 195 



The basal facts of Mendel's rule or principle of inherit- 

 ance have been abundantly established, but their inter- 

 pretation is not so simple, and they cannot be applied 

 to life in general until much more knowledge has been 

 obtained of essential dominant or recessive characters 

 in organisms and their physiological meaning. Neither 

 Darwinism nor Mendelism supplies more than a partial 

 interpretation of the operations of animate Nature, and 

 few biologists would now care to proclaim them as 

 impregnable truths. Whatever may be the ultimate 

 decision as to the determining factors of evolution, we 

 can say that Darwin was the first to describe the sig- 

 nificance of variation, and Mendel to show how the com- 

 plex, threads of heredity could be disentangled. The 

 names of these two. naturalists will be remembered for 

 all time as those of the two greatest contributors to the 

 theory of organic evolution. 



In looking for the cause of evolution as against special 

 creation, the central fact which Darwin set himself to 

 explain was that of adaptation to environment, whereby 

 small differences were accumulated until new forms were 

 produced. No direct evidence as to this modifying 

 factor could be obtained from the geological record, 

 which was also too imperfect to show the transitions 

 from one species to another demanded by the theory. 

 The support which evolution was to receive from the 

 tablets of stone came later. 



At the beginning of the nineteenth century it was well 

 understood by geologists that fossils provide a means of 

 determining the ages of the sedimentary rocks of our 

 globe ; they were accepted as convenient labels or signs 

 of relative antiquity, but little was known of their own 

 lines of development. Gradually, however, the view 

 was formed that there is no sharp distinction between 



